1. Technical Field
The present invention relates to a projector that projects and displays an optical image formed on a light modulator through a projection system on a screen, and more particularly to a projector that allows a viewer to observe a stereoscopic projected image through polarized eyeglasses.
2. Related Art
A technology for stereoscopically representing a displayed image by using a projector capable of large-screen display has been developed and brought into practical use. In stereoscopic image display using a projector, it is typical to use a parallax-based method in which images for the left and right eyes are projected on a screen and the left and right eyes view the respective images. In this process, the viewer needs to choose an image corresponding to each of the eyes. To this end, for example, a polarization-based projector using polarized eyeglasses has been known.
A polarization-based projector simultaneously displays images for the left and right eyes containing parallax information and having different polarization states on a screen. Alternatively, time-course frames are alternately and continuously displayed. When the viewer looks at the displayed images for the left and right eyes having different polarization states through polarized eyeglasses having polarization selectivity, each of the eyes selectively views only the corresponding one of the two types of image, whereby the displayed images can be visually recognized stereoscopically.
To simultaneously display images for the left and right eyes having polarization states different from each other on a screen, a method using two projectors has been proposed (see JP-A-9-54375). In the method, a first projector displays an image for the left eye and a second projector displays an image for the right eye. In the method using two projectors, it is difficult to match the brightness, color tone, and other characteristics of displayed images and adjust projection positions between the two projectors. Further, using two projectors is problematic in terms of size reduction and usability of the system.
On the other hand, the method in which images for the left and right eyes having polarization states different from each other are alternately and continuously displayed in a time division manner is advantageous in that only a single projector allows displayed images to be stereoscopically represented. A projector based on this method has also been proposed (see JP-A-63-18894).
In the stereoscopic image display apparatus described in JP-A-9-54375, inconvenience of having to prepare two projectors that project light fluxes having different polarization states is solved as follows: Two projectors that project light fluxes having the same polarization state are used, and a polarization switching device formed of a reflection mirror changes the polarization state of the light projected by one of the projectors. Images for the left and right eyes having different polarization states are thus produced. On the other hand, the projector described in JP-A-63-18894 alternately displays images for the left and right eyes in a time division manner by projecting light from a liquid crystal light valve via a polarization switching device formed of a liquid crystal panel that alternately switches the polarization state of the light. Both the apparatus need to have a device for switching the polarization state of projection light between a light modulator and a screen.
In addition to the apparatus described above, a DLP projector (DLP is a registered trademark of Texas Instruments Inc.) using a minute mirror-based light modulator having a large number of minute mirrors arranged in a matrix has been brought into practical use. The minute mirror-based light modulator, which rewrites image data by using a frame sequential method, keeps displaying an image corresponding to a frame for a fixed period, then instantaneously replaces the image with a new image corresponding to the next frame, and displays the new image for a fixed period. That is, the minute mirror-based light modulator cannot rewrite image data during each image display period. To allow the DLP projector to display a stereoscopic image, the polarization switching device described above is provided and used to switch the polarization states of projection light fluxes simultaneously across an image display area in synchronization with the timing at which the minute mirror-based light modulator instantaneously rewrites image data.
On the other hand, a liquid crystal projector including a light modulator using a liquid crystal material has been brought into practical use. A typical liquid crystal light modulator does not have an image memory that holds image data and covers the entire pixels but rewrites the image data by using a line sequential method, that is, a method for sequentially scanning one or more lines (sometimes referred to as a scan line) from one side to the other. In the liquid crystal projector, a one-frame image is taken as a set of line-shaped images, which are sequentially rewritten on a line basis in the period during which the one frame is displayed. A new image corresponding to the next frame is thus produced. That is, a one-frame image is a mixture of image data corresponding to the current frame and new image data corresponding to the next frame with a scan line interposed therebetween.
A polarization switching device used in a liquid crystal projector therefore needs to switch the polarization state of projection light by using a line sequential method in accordance with the liquid crystal light modulator that writes image data by using a line sequential method. That is, a light transmissive area of the polarization switching device is segmented into a plurality of line-shaped areas, and the polarization state of light passing through each of the areas is switched individually. The polarization switching device also needs to include a polarization state switching device in each of the areas or externally include a scan circuit for sequentially selecting each of the areas and switching the polarization state of light passing therethrough. A polarization switching device of this type is hereinafter referred to as a “segmented-area polarization switching device.”
However, since projection light having exited from the liquid crystal light modulator is divergent light, a segmented-area polarization switching device simply disposed immediately downstream of the liquid crystal light modulator disadvantageously receives the projection light whose diameter has increased after it had exited from each pixel in the liquid crystal light modulator. To address the problem, it is conceivable to increase the number of segmented areas, each of which switches the polarization state of the projection light, in the segmented-area polarization switching device. In this case, however, the positions where image data are written in the liquid crystal light modulator still do not match the positions where the polarization states are switched in the segmented-area polarization switching device with precision of at least approximately one line (row) or several to several tens of lines in the pixel arrangement. As a result, for example, part of an image for the left eye has a polarization state that should not present in the image for the left eye (but should present in an image for the right eye) or suffers from other unwanted phenomena. The polarization state of a projected image is thus locally disturbed, and the local portion does not provide an appropriate stereoscopic viewing state, resulting in degradation in image quality.
The above description has been made with reference to the line sequential method, which is a typical scheme of writing image data in a liquid crystal light modulator. In some cases, a liquid crystal light modulator sequentially rewrites image data on a pixel basis, in other word, performs what is called point sequential writing. The problem described above also occurs in a projector based on the point sequential method.